Abstract
This research work assesses the influence of a chromium-rich waste (from potassium dichromate manufacture) on the hydration and hardening processes of two types of Portland cements with limestone filler and slag additions. Therefore, mixtures of Portland cement and chromium-rich waste, corresponding to 0.5% wt. and 1% wt. Cr, were prepared and tested. The analyses performed on cement pastes with chromium waste content, showed that chromium immobilization is mainly due to the formation of Ca6Al2Cr3O18�32H2O (CrEt); this compound results by the substitution of [SO4]2- groups from ettringite lattice of with [CrO4]2-. CrEt crystals growth on the surface of clinker particles forms a diffusion barrier which explains longer setting times for cements with chromium content. The increase of chromium content in the studied systems decreases the compressive strength values but these remain above the lower limits imposed for this type of materials. The chromium content in leachates prepared according to the method described in SR EN 12457-2, was well below the legal limit of 70 mg/Kg established by Romanian legislation. A better chromium immobilisation was achieved in the cement with slag content, in good correlation with the nature and amount of formed hydrates.
Highlights
In cement industry various types of waste can be recycled as secondary materials with positive impact on the environment protection as well as on the energy consumption [1–6]
Chromium immobilization in the studied cements was estimated considering the chromium concentration of a leachate formed after the immersion, for 24 hours, in distilled water of the granular material resulted by the crushing of cement pastes hardened for 28 days; the samples preparation was made in accordance with the method presented in SR EN 12457-2 [26]; the chromium concentration in leachate was determined by inductively coupled plasma - optical emission (ICP-OES) technique with a transcribed apparatus conductively coupled plasma optical emission spectrometer
- for chromium-rich waste, at 1414 cm-1 and 870 cm-1 the absorption bands are attributed to the CO32- bonds from Ca(Mg)CO3; the band from 870 cm-1 is attributed of the CrO42- bonds from the CaCrO4.2H2O; at about 716 cm-1 the shoulder can be correlated with the presence of SiO2 in waste;
Summary
The materials used for this study were: i) two types of Portland cements: - CEM II / A-M (S-LL) – with the following components: clinker (76.6%); limestone (9.9%); blast furnace slag (5.1%); siliceous addition (2.6%); setting regulator (5.9%);. - CEM II / A-L (LL) - with the following components: clinker (86.9%); limestone (5.7%); siliceous addition (2.5%); setting regulator (4.9%). Ii) a chromium-rich waste resulted in the potassium dichromate manufacture. The main characteristics of this waste were reported in a previous study [21]. The main crystalline phases assessed by XRD in the chromium waste were: calcium chromium oxide hydrate (PDF 37-1367), calcium hydroxide (PDF 84-1270), calcium carbonate (PDF 050586) and magnesium hydroxide (PDF 44-1482) [21]. Chromium waste was dosed to yield 0.5 and 1 wt. % chromium content in cement
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